This invention relates to CDMA (Code Division Multiple Access) communication system and, in particular to multi-user interference cancellor which is installed in a receiving apparatus of the system and is adapted to cancel mutual interference among user-signals.
In the CDMA communication system, a transmitter spreads spectrums of a user signal with particular codes which have transmission rates that are much higher than the user signal, to produce a spread user-signal. Such a spread user-signal is transmitted through transmission paths on which base stations, exchanges, or the like, are existing.
Generally, multiple users participate in the CDMA communication system so that spread user-signals of multiple users are multiplexed on the transmission path. Herein, such a multiplexed signal is referred to as a multi-user spread-signal. In the multi-user spread-signal, interference occurs due to mutual correlation in accordance with user-signals. The interference decreases the quality of the communication and therefore, it is undesirable.
To remove the influence of the interference, the receiving apparatus of the system has an interference cancellor. In a popular way, the interference cancellor produces symbol replica signals corresponding to the user signals, by using propagation characteristics of the transmission paths, the codes responding to the user signals, and so on. Then, to extract a particular user signal, the interference cancellor repeatedly subtracts, from the multi-user spread-signals, spread replica signals obtained by spreading the symbol replica signals except for one symbol replica signal corresponding to the particular user. For the repeated processing, the interference cancellor generally has a plurality of interference canceling stages each of which executes one subtraction process as mentioned above. For example, such a technique is disclosed in Japanese Patent Laid-Open (JP-A) No. 10-117180.
Also, the above interference cancellor has been applied to extract all user-signals at the same time. Such an application of the interference cancellor is called a multi-user interference cancellor and, for example, is disclosed in Japanese Patent Nos. 2737775, 2737776, and 2746261, and JP-A No. 10-51353. Generally, these multi-user interference cancellors have high calculation loads and are large sized circuits, due to the extraction of all user-signals. Therefore, research has been directed to decreasing the calculation quantity and shrinking the circuit""s size while maintaining accuracy.
However, any proposals have not yet provided a sufficient scheme. One proposal provides small sized circuits but has a low accuracy. Another proposal has a high accuracy but provides large sized circuits and much quantity of the calculation.
This invention, therefore, provides a multi-user interference cancellor having a high accuracy, shrinked circuits and a little quantity of the calculation.
According to one aspect of the present invention, in response to a multi-user spread signal having a plurality of spread user-signals, a multi-user interference cancellor produces user-signals in correspondence with the spread user-signals. Such cancellor comprises a plurality of interference canceling stages. Each of the interference canceling stages reproduces symbol replica signals corresponding to the user-signals in response to an input signal of the interference canceling stage, in question. And also, the interference canceling stage produces a remnant component of the multi-user spread signal as an output signal of the interference canceling stage, in question. For example, such interference canceling stages may be formed by first, middle, and last interference canceling stages which are successively coupled in order.
Herein, each of the interference canceling stages except for the last one comprises a plurality of interference estimation units (IEUs), a processor, and a subtracter.
The IEUs are equal in number to processable user-signals. The IEUs produces the symbol replica signals corresponding to the user-signals assigned to the IEUs, and also, produces local spread signals having result information of processing in only the IEUS, in question, respectively. Herein, the local spread signals are, for example, first spread signals in first interference canceling stage, and middle spread signals in middle interference canceling stage.
The processor processes the local spread signals to calculate, from them, a single vector signal which is obtained by a combination of the local signals. For example, the processor may comprise a multiplexer, an amplitude limiter, and a filter. The multiplexer multiplexes the local spread signals at every IEUs to produce a multiplexed signal. The amplitude limiter limits an amplitude of the multiplexed signal to a processable range of the cancellor, in question, to produce an amplitude limited signal. The filter filters the amplitude limited signal into a filtered signal as the single vector signal.
The subtracter subtracts the single vector signal from the input signal, and thereby, to produce the output signal representative of the remnant component.
Furthermore, the last interference canceling stage produces the user-signals from the symbol replica signals and the output signal all produced by a preceding one of the interference canceling stages.